A copper plating process is widely used in the fields of semiconductor devices and printed wiring. A plating apparatus for performing the copper plating process will be described with reference to FIG. 13. As shown in FIG. 13, the plating apparatus includes a plating bath 100 for holding a plating solution therein, an anode 101 immersed in the plating solution retained in the plating bath 100, an anode holder 102 holding the anode 101, and a substrate holder 103 for holding a substrate W. The substrate W and the anode 101 are disposed in the plating solution in the plating bath 100 such that they oppose each other. The anode 101 is coupled to a positive pole of a power source 104, while the substrate W is coupled to a negative pole of the power source 104. When a voltage is applied between the anode 101 and the substrate W, an electric current flows from the anode 101 through the plating solution to the substrate W, whereby the substrate W is plated. Such an electric current, flowing from the anode 101 through the plating solution to the substrate W, will be hereinafter referred to as a forward electric current.
A soluble anode, which is composed of, e.g., phosphorus-containing copper, may be used as the anode 101. However, the use of such a soluble anode can cause a deposition of a black film on a surface of the anode 101. Since it is difficult to appropriately perform plating of the substrate W while controlling the deposition of the black film, a defect may be produced in a copper film formed on the surface of the substrate W.
FIG. 14A is a schematic view showing a cross section of the surface of the substrate W. As shown in FIG. 14A, a recess 110, which is called via-hole, is formed in a surface of a dielectric film 109. Further, a conductive layer 114, such as a seed layer, is formed on a flat surface of the dielectric film 109 and on a surface of the recess 110. In a process of depositing metal in the recess 110 having a high aspect ratio, such as TSV (Through Silicon Via), an additive, including a suppressor for suppressing the deposition of the metal, is used to prevent the deposition of the metal in an area around an opening of the recess 110. However, when a soluble anode containing copper is used, the copper dissolves in the form of monovalent copper ions (C+), which combine with the additive, thus reducing the additive.
In view of this, in recent years, an insoluble anode tends to be used instead of the soluble anode. When copper plating is carried out using an insoluble anode as the anode 101, copper ions are reduced on the substrate W and a copper film is formed on the substrate W, while electrolysis of water occurs on the anode 101, whereby oxygen is generated. When the application of the voltage is stopped, an electric current flows from the anode 101 to the substrate W via the power source 104 as shown in FIG. 13, because the substrate W and the anode 101, which are composed of different metals, have different spontaneous potentials in the plating solution. Thus, the electric current flows in a direction opposite to the direction of the electric current that flows during plating of the substrate W, resulting in dissolution of the copper film formed on the substrate W. Such electric current, flowing in the opposite direction, will be hereinafter referred to as a reverse electric current.
A magnitude of the reverse electric current depends mainly on a concentration of dissolved oxygen in the plating solution and is likely to increase especially when the concentration of dissolved oxygen is high immediately after plating. The reverse electric current is likely to flow when the insoluble anode is used which causes a large potential difference between the insoluble anode and the substrate W. In contrast, the reverse electric current hardly flows when the soluble anode is used which is composed of phosphorus-containing copper, i.e., the same kind of metal as the substrate W.
Copper dissolves as monovalent copper ions (C+) or divalent copper ions (C2+) in the plating solution. The monovalent copper ions are unstable and are therefore immediately oxidized into divalent copper ions. If an accelerator (e.g., SPS) for accelerating deposition of a metal is included in additives, the accelerator is reduced in a reaction to oxidize the monovalent copper ions (C+), whereby characteristics of the additives change. For example, when SPS is used as the accelerator, SPS will be reduced into MPS.
A balance between the suppressor and the accelerator is of importance in order to allow a metal to deposit preferentially from a bottom of the recess 110, i.e., to effect so-called bottom-up growth of the metal. If the reverse electric current flows and the additive (accelerator) is reduced, the balance between the suppressor and the accelerator will be lost, resulting in a formation of a defect, such as a void, in the recess 110.
As described above, if the reverse electric current flows between the substrate W and the anode 101, SPS will be reduced and MPS will be present in excess in the plating solution. If NIPS, having a strong effect to accelerate copper plating, is present in excess in the plating solution, an area where plating should be suppressed by the suppressor will be plated, resulting in a formation of a defect, such as a void, in the recess 110. Thus, in that case, MPS behaves as an electrolyte component that inhibits bottom-up plating on the substrate W.
In an example illustrated in FIG. 14B, a photoresist 111 is formed on the conductive layer 114. When the voltage is applied between the substrate W and the anode 101, copper 112 is deposited in an opening of the photoresist 111 and in the recess 110. However, if the balance between the suppressor and the accelerator is lost, the bottom-up growth of copper 112 is not achieved and a depression 113, which is called a dimple, may be formed in the copper 112 lying above the recess 110.
Also in a process of depositing copper 112 on a substrate W which does not have the photoresist 111 as shown in FIG. 14C, a surface of the copper 112 may not be flat after plating and a depression 113 may be formed. Moreover, a void may be formed in the recess 110.
In an example illustrated in FIG. 14D, the conductive layer 114 is formed on an underlying interconnect 115, and the photoresist 111 is formed on the conductive layer 114. Also in a plating process of depositing copper 112 in the opening of the photoresist 111 (the formation of a bump), the effect of the additive may be inhibited, resulting in a formation of irregularities in the surface of the copper 112 or a formation of a bump having an abnormal shape.
In this specification, the bottom-up plating refers to a plating technique for achieving a desired morphology in a to-be-plated area as shown in FIGS. 14A through 14D. Specifically, the bottom-up plating refers to a plating technique which effects gradual progress of plating from a bottom of an opening or a bottom of a recess surrounded by a dielectric material (including a photoresist). A failure to achieve the bottom-up plating will result in, for example, an abnormal shape of deposited metal or the formation of void in deposited metal.